1. Field of the Invention
The invention refers to a refractory gas permeable bubbling plug for a vessel for receiving molten metal with at least one measuring probe for displaying the remaining size of the plug, the tip of said measuring probe being arranged in a predefined distance from the cold end of the plug.
2. Description of the Prior Art
It is common practice to introduce various kinds of gases through the refractory lining of vessels into the molten metal. This is done in converters for steel refining, in casting ladles, in tundishes for continuous steel casting and in other types of metallurgical furnaces and vessels. Gas bubbling has a stirring effect in the molten metal or can also give rise to various metallurgical reactions. For this purpose, refractory gas permeable elements are inserted into the refractory lining, most frequently in the bottoms, of furnaces and vessels, said elements being commonly described as bubbling plugs.
Bubbling plugs may either comprise a refractory brick body of high porosity, with the gas flowing through the open pores between the refractory grains, or they may comprise a refractory brick body of low or no porosity, the brick being provided with slots or channels which allow the passage of gases, the latter being referred to as bubbling plugs with aligned porosity.
As a consequence of the interaction between molten metal and the introduced gases, the bubbling plugs are in general subject to higher wear than the surrounding refractory lining. Although there is the tendency to adjust the durability of the bubbling plugs to that of the surrounding lining by choosing an especially wear-resistant refractory material, it is, however, ofter more economical to change the bubbling plugs once or several times during one furnace campaign of the refractory lining.
In order to recognize the correct moment either for changing the bubbling plug or for replacing the refractory lining, it is desirable to arrange a device to the bubbling plug which indicates that the bubbling plug has reached a predefined remaining size.
It has been tried before to optically evaluate the remaining size of the bubbling plug by supervising its hot face. For example, in a bubbling plug having a cross section which expands conically or step-wise in the direction of the cold side, the degree of wear may be derived from the increase in the visible diameter. According to German patent DE-C-31 42 989, the bubbling plug consists of at least two refractory bodies of varying light emisivity, said bodies preferably being intertwined and comprising separation layers in a predefined height of the plug. Whenever wear causes one of said layers to be reached, this shall be recognized by a change of the brightness of the light radiating from the hot surface. However, such optical methods are very unreliable, as an even surface of the bubbling plug--which is mostly not the case--is necessary for precise supervision.
Also electrical display devices are known for supervising wear. According to European patent EP-B-0 082 078, the bubbling plug comprises a plurality of electrodes located in a graduated manner within the height of the plug and insulated from one another, the circuit between said electrodes being closed by the molten metal because of its electric conductivity, said circuit then being displayed. As the contact between the molten metal and the electrodes may easily be interrupted, e.g. by slag inclusions, this arrangement is very susceptible for defects. In order to avoid the above mentioned disadvantage, according to published German patent application DE-A-34 24 466, two electrodes are arranged in a probe which is included in the bubbling plug and is closed towards the hot end of the plug, the tip of the probe being arranged in a certain height of the plug. If the wear of the plug has progressed near to said height of the plug, the ends of the electrodes melt together by the increase in the temperature and thus causing the electric circuit of the display to be closed. According to German patent DE-D-35 03 221, the bubbling plug comprises electric conductors which are arranged in a framework of longitudinal conductors and transverse conductors, the latter being arranged in a graduated manner within the height of the plug. In the unused plug the display circuit is closed. During operation the electric conductor will melt and be interrupted by the increasing approach of the molten metal as a consequence of the increasing wear. According to a variation, the bubbling plug may comprise thermocouples instead of said conductors, the thermocouples being located in different heights of the plug and being connectable to an electric display unit.
In practice the above-mentioned electric display units did not bring about reliable values, as electric connections are very susceptible for defects during the rough operations in a steel plant and especically in the case of moveable vessels, e.g. casting ladles.
The bubbling plug according to German Patent DE-C-36 23 609 comprises in the gas introductory opening a valve with a valve body which is held in the open position by a melting fuse or a bimetallic strip. When the temperature increases as a consequence of wear of the bubbling plug, the melting fuse will melt or the bimetallic strip will be deflected, whereby the valve body will change to a closing position and in which a lower amount of gas will flow through. As in this arrangement the melting fuse or the bimetallic strip are located in some distance away from the molten metal, an unobstructed flow of heat is necessary for a reliable triggering, which, however, might nevertheless be obstructed.